在C#/.NET中，将文本内容写入文件最简单的方法是调用 File.WriteAllText() 方法，但这个方法没有异步的实现，要想用异步，只能改用有些复杂的 FileStream.WriteAsync() 方法。

使用 FileStream.WriteAsync() 有2个需要注意的地方，1是要设置bufferSize，2是要将useAsync这个构造函数参数设置为true，示例调用代码如下：

public async Task CommitAsync(){    var bits = Encoding.UTF8.GetBytes("{\"text\": \"test\"}");    using (var fs = new FileStream(        path: @"C:\temp\test.json",         mode: FileMode.Create,         access: FileAccess.Write,         share: FileShare.None,         bufferSize: 4096,         useAsync: true))    {        await fs.WriteAsync(bits, 0, bits.Length);    }}

看这个方法的帮助文档中对useAsync参数的说明：

//   useAsync:        //     Specifies whether to use asynchronous I/O or synchronous I/O. However, note        //     that the underlying operating system might not support asynchronous I/O,        //     so when specifying true, the handle might be opened synchronously depending        //     on the platform. When opened asynchronously, the System.IO.FileStream.BeginRead(System.Byte[],System.Int32,System.Int32,System.AsyncCallback,System.Object)        //     and System.IO.FileStream.BeginWrite(System.Byte[],System.Int32,System.Int32,System.AsyncCallback,System.Object)        //     methods perform better on large reads or writes, but they might be much slower        //     for small reads or writes. If the application is designed to take advantage        //     of asynchronous I/O, set the useAsync parameter to true. Using asynchronous        //     I/O correctly can speed up applications by as much as a factor of 10, but        //     using it without redesigning the application for asynchronous I/O can decrease        //     performance by as much as a factor of 10.

从中可以得知，只有设置useAsync为true，才真正使用上了异步IO。